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Chapter 6: Calibration Using Free In-air Measurements
6.1. General
The free in-air measurement technique cannot be used for low-energy
sources due to air-kerma calibration factor uncertainties, low air-kerma rates,
and uncertainties due to air humidity. For long-lived radionuclides, e.g.,
137
Cs,
a source can be a working standard.
6.2. Formalism for reference air kerma rate
K
R
reference air kerma rate
N
K
chamber air kerma calibration factor at desired photon energy
M
U
measured charge, corrected for T, P, recombination, transit error
t
time for collecting charge
k
scat
correction for room scatter
k
n
correction factor for non-uniform electron fluence within the cavity
d
measurement distance from source center to chamber center
d
ref
reference measurement distance (i.e., 1 meter)